The present invention relates to hydraulic lift mechanisms having a cushion or buffer feature, and more particularly to a vertically adjustable chair employing a cushioned hydraulic lift mechanism.
Past hydraulic lift mechanisms in the form of piston rod and cylinder combinations which include a series of axially spaced outlet orifices in the cylinder wall are generally known. Many different designs of such mechanisms exist in the prior art and employ a series of axially spaced outlet ports or orifices for similar reasons. One previous design is shown in U.S. Pat. No. 4,065,112 issued to Leskovec et al. and entitled Hydraulic Jack Cushioning Apparatus. Leskovec et al. show axially spaced orifices formed in the lower end of a cylinder wall. The cylinder telescopically receives a piston which sequentially passes the axially spaced orifices at the lower end of the piston stroke to provide a cushioning or decelerating effect on the piston. Under the weight of the load being lifted, the piston initially descends at a relatively rapid rate, however, this rate of descent decreases each time the lower edge of the piston covers or blocks another axially spaced orifice. Blockage of successive orifices restricts or slows down the hydraulic fluid flowing out of the cylinder and thereby slows the rate of descent of the piston.
Other examples of hydraulic deceleration devices employing similar concepts to those shown by Leskovec et al. are disclosed in U.S. Pat. Nos. 1,575,973 to Coleman and 3,491,993 to Scholin et al. Each of these patents similarly disclose piston and cylinder combinations in which the cylinder wall contains a plurality of axially spaced orifices which are successively closed off by the piston to thereby provide a decelerating effect on the piston. Each of these prior designs which utilize a plurality of axially spaced orifices provide a deceleration or cushioning effect which is nonlinear. That is, the piston will decelerate at a rate corresponding to the number of orifices left uncovered and will abruptly change to a new rate of deceleration as soon as another orifice is blocked by the piston. The effect of this nonlinear deceleration is that the piston will not smoothly decelerate to a stop but will instead descend through a series of bumping or jolting motions.
Many different designs of lift chairs used for patient examination and other purposes are also known in the prior art. These chairs employ hydraulic, electric, or other means for allowing the chair to be moved vertically, for example, during the examination of a patient by a medical professional. Hydraulic lift mechanisms have been used in the past by rigidly securing the chair to a piston rod assembly which is hydraulically operated within a cylinder at the base of the chair. One prior method of decelerating the piston of a hydraulic chair lift has been to use a pin at the lower end of the piston which fits into a slightly larger outlet port at the lower end of the piston stroke so as to restrict the outflow of hydraulic fluid from the cylinder and thereby decelerate the piston and the chair at the lower end of the stroke.
Although this known method of decelerating a vertically adjustable chair as it approaches its lowermost position successfully prevents the abrupt stop at the lower end of the stroke, it also presents certain undesirable features. First, the pin at the lower end of the piston forced the chair to be mounted at a greater than desirable height. Second, and more importantly, the pin caused the fluid outlet to go from an unrestricted state to a fully restricted state as soon as the pin entered the slightly larger outlet port. Although this sudden deceleration is more comfortable, for example, than having the piston "bottom out" at full speed, it is still felt as an abrupt stop by the person sitting in the chair.
Accordingly, there is a need for a hydraulic cushion which provides for smooth deceleration and cushioned stop at the lower end of the stroke and further for hydraulically operated lift chairs incorporating such a cushioning or deceleration feature.